Reverse crunches! Also, i've noticed that standing ab exerices tend to work both the upper and lower abs as well. Such as:
works abs, obliques
Stand with feet hip-width apart, knees slightly bent, hands clasped behind head and elbows pointed out to sides. Lift right leg to side, knee bent, thigh parallel to floor. Keeping knee lifted, bend torso to right side, bringing elbow and knee toward each other.
Side leg extension
works abs, legs, butt, back
Lie faceup with arms at sides, palms down. Bend right knee 45 degrees and rest leg on ball with hips off floor, abs engaged, left leg extended toward ceiling. Lower left leg across body as far as you comfortably can, toes pointed, keeping shoulders on floor (as shown). Slowly return to start. Do 10 reps; switch legs.
works lower abs and hips
Start the V-Up by lying flat on a mat and then raise both your legs and arms together while keeping them as straight as possible. Keep your lower back as straight as possible as well. Lower down to the starting position slowly while exhaling